Stream-wise thermal gradient cloud condensation nuclei chamber

ABSTRACT

A cloud condensation nuclei instrument including an aerosol flow line ( 320 ) to carry a desired sample to the CNN chamber ( 310 ) and a temperature control unit ( 313 A,  313 B) to provide a monotonically increasing temperature profile along the direction of flow.

This application claims the benefit of U.S. Provisional Application No.60/411,688 entitled “MEASUREMENT OF CLOUD CONDENSATION NUCLEI USINGLONGITUDINAL THERMAL-GRADIENT CHAMBER” and filed on Sep. 18, 2002 byGregory C. Roberts.

BACKGROUND

This application relates to aerosol measurements, and more particularly,to measurements of cloud condensation nuclei.

The effect of human activities on climate is being recognized as one ofthe most important issues facing society (International Panel of ClimateChange, 2001). Humans influence climate in numerous ways by cooling orheating the planet. Some components (such as greenhouse gas warming) arewell understood and quantified; others are subject to high uncertainty.Aerosols (airborne particulate matter) belong to the latter category. Itis believed that aerosols have a net cooling effect, but quantitativeestimates are highly uncertain, of the order of the greenhouse warmingeffect itself. This uncertainty primarily originates from poorlyunderstood aerosol-cloud interactions. Aerosols are the seeds for cloudformation, and those, around which droplets form, are called cloudcondensation nuclei (CCN). The size, concentration, and affinity towater vapor of CCN can directly influence the size and concentration ofcloud droplets.

Increasing the concentrations of aerosols (which occurs under pollutedconditions) leads to more reflective and persistent clouds. Since cloudsare very effective reflectors of incoming solar radiation, even smallperturbations in their properties can significantly decrease the amountof solar radiation absorbed by the climate system, and thus lead tocooling, otherwise known as the aerosol “indirect effect”. Of all thecomponents of climate change, the aerosol indirect effect is the mostuncertain and potentially with the largest cooling effect. Until theaerosol indirect effect is well quantified, society is incapable ofassessing its impact on future climate.

Measurements of CCN are fundamental for providing the link between cloudmicrophysics and the physical and chemical properties of aerosol. It isthis liaison that is essential to improving our understanding ofaerosol-cloud interactions and their subsequent effect on climatethrough modification of cloud radiative properties and the hydrologicalcycle. CCN measurements, however, are among the most challengingmeasurements in atmospheric sciences as obstacles in instrumentaldevelopment and nuances in interpreting the data pose inherent problems.The primary source of problems lies in clouds themselves; they form inregions of very low water vapor supersaturation (by most a few tenths ofa percent). Developing a technique that generates low supersaturation ina controlled manner, within in an ultralight package that respondsquickly to ambient changes (necessary conditions for in-situ aircraftmeasurements), has proven to be challenging.

In addition, instrument development in this field has been largelyempirical. As a result, measurements were often subject to unquantifieduncertainty. Significant improvements in the measurement techniques areneeded and this development constitutes an important step in thisdirection.

The ability of a particle to nucleate is at least in part determined bythe saturation level of the environment, the size of the particle, andthe chemical composition of the particle. When the relative humidityexceeds the saturation level where the vapor phase and the liquid phaseare in equilibrium, a supersaturation state establishes and vapor beginsto condense on surfaces and some particles. At a certain criticalsupersaturation, when the diameter of a condensation nucleus of a givenchemical composition exceeds a critical diameter, the nucleus is said tobe “activated.” Upon this activation, vapor can condense spontaneouslyon that nucleus and cause the nucleus to grow to a very large size whichis limited only by the kinetics of condensational growth and the amountof vapor available for the condensational growth. The critical diameterat a given supersaturation usually changes with the chemical compositionof the particles. Hence, particles of different chemical compositionscan become activated at different sizes. One way to characterizecondensation nuclei is to measure the critical supersaturation at whicha particle activates. Various cloud condensation nucleus spectrometershave been developed for producing and measuring supersaturations in adesired range.

It is generally understood that cloud formation is determined by asubset of aerosol particles that grow into droplets by heterogeneouswater nucleation. The ability of an aerosol particle to serve as CCNdepends primarily on its size and soluble mass. The ratio of water vaporpressure at the surface of the droplet to that of a flat plane is theequilibrium saturation ratio S_(R) ^(eq), and is described by the Köhlertheory initially published by Köhler, “The nucleus in and the growth ofhygroscopic droplets,” Trans. Faraday Sot., 32, 1152-1161 (1936). Seealso, e.g., Pruppacher and Klett, Microphysics of Clouds andPrecipitation, Kluwer Academic Publishers, Boston, (1997) and Seinfeldand Pandis, Atmospheric chemistry and physics: From air pollution toclimate change, 1326 pp., John Wiley, New York (1998). Two competingterms describe the Köhler equation; the surface tension term (i.e., theKelvin effect) accounts for enhanced vapor pressure due to dropletcurvature and scales to inverse diameter, D_(p) ⁻¹, and the dissolvedsolute term (i.e., the Raoult effect) depresses the water vapor pressureat the droplet surface and scales to D_(p) ⁻³. The maximum S_(R) ^(eq)of the Köhler curve defines the critical supersaturation, S_(c), andoccurs at the droplet's critical diameter, D_(pc). The droplet is instable equilibrium with its environment when its diameter is less thanD_(pc). However, once the particle has activated (i.e., D_(p)>D_(pc)),the particle will continue to grow as long as the surrounding vaporpressure of water in the air is greater than the equilibrium vaporpressure of the solution droplet. The saturation ratios are oftenexpressed as supersaturations, S_(v), in percent (i.e., S_(v)(%)=(S_(R)−1)×100%).

The shape of the Köhler curve dictates droplet growth and can be readilymodified by surfactants and slightly soluble constituents in ambientaerosols. The presence of surface-active substances, such aswater-soluble organic carbons (WSOC), can have a significant influenceon the equilibrium vapor pressure by reducing the droplet's surfacetension, which lowers S_(c) and enhances droplet growth. Slightlysoluble compounds and soluble gases also affect the shape of the Köhlercurve and may even allow the occurrence of stable, unactivated dropletsof about 20 μm diameter in realistic, albeit polluted, conditions. Suchmodifications to the Köhler curve result in different growth rates ofdroplets and may impose challenges in defining activated and unactivateddroplets and what constitutes CCN. Nonetheless, interpretingmeasurements from CCN instruments requires an understanding of thesenuances and proper assessment of their importance in light of theparticular experiment's focus.

SUMMARY

This application includes, among others, exemplary implementations ofcontinuous-flow CCN chambers with improved measurement accuracy based ona monotonic thermal gradient in the stream-wise direction of the airflow. Such CCN chambers can be used for real-time, in-situ measurementsof CCN and may be configured to operate at a high sampling ratesufficient for airborne operations. Direct measurements in theclimatically important range of supersaturations of less than 0.1% arepossible with the present CCN chambers.

The exemplary continuous-flow CCN chambers described here have amonotonic thermal gradient in the stream-wise direction of the air flowin the CCN chambers. Hence, the temperature of the CCN chamber changesmonotonically along the flow direction from the input end to the outputend. In general, the temperature may monotonically increase along theflow direction. In one implementation, for example, the temperaturewithin the CCN chamber may linearly increase from the input end to theoutput end of the CCN chamber along the steam-wise direction. In acylindrical CNN chamber, such a linear spatial temperature gradient canproduce a quasi-uniform supersaturation along the flow direction. Thisquasi-uniform supersaturation along the flow direction can maximize thegrowth rate of activated droplets in the flow and thus significantlyenhance the instrument performance. The temperature gradient and theflow control the supersaturation in the CCN chamber and may be adjustedor modified to retrieve the CCN spectra.

In one implementation, a CCN device includes a cloud condensation nucleichamber having an input to receive an aerosol flow, a region ofsupersaturation to grow cloud condensation nuclei, and an output toexport the aerosol flow. A thermal control is engaged to the chamber toproduce a monotonic thermal profile in a stream-wise direction of theaerosol flow from the input to the output in the chamber.

In another implementation, a cloud condensation nuclei measuringapparatus has a chamber to receive an air sample and to keep the airsample in a region of supersaturation within a specified range, aheating system providing an increasing temperature gradient along theaxis of the chamber in the direction of flow, and a particle countercoupled to the chamber to measure particles in the air sample output bythe chamber and to provide a count indicative of particles within aselected size range.

In a more specific implementation, a CCN instrument is provided in whichan air sample is introduced in the center of a vertical, cylindricalcolumn whose surfaces are wetted. This configuration keeps the sample ina region of nearly uniform supersaturation and minimizes wall losses.The air then flows vertically downwardly through the chamber, where CCNsactivate and grow into droplets. An optical particle counter at theoutlet of the chamber detects all particles having diameters over athreshold such as 0.5 microns. Particles above a next threshold, forexample, 1.0 microns are considered CCNs, and their total per unitvolume comprises the CCN concentration. A monotonically increasingtemperature gradient is provided along the axis of the chamber in thedirection of flow.

These and other implementations and associated methods are described ingreater detail with reference to the drawings, the detailed description,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows computed saturation and thermal gradient in analternating-gradient CCN chamber for repeating hot/cold sections in acylindrical symmetry. The upper graph illustrates the contours of thesupersaturation profile in the radial, r, and streamwise, z, dimensions.The centerline is at r=0. The lower graph shows the alternatingtemperature gradient (bars) and the development of the supersaturationprofile near the centerline of the chamber (multiple oscillating lines).The dashed line denotes the transition between an undersaturated(ratio<1) and supersaturated (ratio>1) region of the instrument. Noticethat the minimum supersaturation ratio briefly drops below 1 (<0%supersaturation) just after the cold sections. Droplets momentarilyevaporate during this period.

FIG. 2 shows saturation and thermal profiles of a linearthermal-gradient CCN chamber in a cylindrical symmetry. The upper graphillustrates the contours of the supersaturation profile in the radialand streamwise dimensions. The centerline is at r=0. The lower graphshows the linear temperature gradient (line with dot symbol) and thedevelopment of the uniform supersaturation profile near the centerlineof the chamber. Notice that unlike FIG. 1, the supersaturation is alwayspositive (or ratio>1) throughout most of the instrument.

FIG. 3 shows one example of a CCN chamber described in this application.

FIG. 4 shows the internal structure that enables annular flow throughthe column that is used in the exemplary CCN chamber in FIG. 3.

FIG. 5 shows predictions of supersaturation based on simplified andfully-coupled model simulations for individual variables at conditionssimilar to normal operation. The error bars on the fully-coupledsimulations represent one-sigma variations to the mean centerlinesupersaturation.

FIG. 6 shows simulations of streamline deviations in the chamber as afunction of column radius for a range of flow rates and temperaturegradients.

DETAILED DESCRIPTION

The specific techniques and designs of continuous-flow CCN chambers witha monotonic thermal gradient in the stream-wise direction of the airflow are now described below as examples. These examples includeimplementations of continuous-flow CCN chambers employing a noveltechnique of generating a supersaturation along the streamwise axis ofthe chamber. Such a CCN instrument may establish a constant temperaturegradient in the direction of flow to explore the differences indiffusion between water vapor and heat so that a quasi-uniformsupersaturation at the centerline can be maintained. As described below,this quasi-uniform supersatuation is desirable in promoting continuousgrowth of the activated particles or droplets throughout the CCNinstrument and thus improving reliability of the measurements.

The streamwise thermal-gradient CCN chambers described here can generatea well-defined supersaturation to simulate cloud-formation in acontrolled environment. The some notable features of these CCN chambersinclude:

-   1. temperature gradient in the streamwise direction generates the    supersaturation by exploiting the difference in diffusion between    heat and water vapor.-   2. continuous flow allows fast sampling (1 Hz measurements), which    is suitable for airborne measurements.-   3. supersaturation is nearly constant at the centerline (for a    constant and increasing temperature gradient), which maximizes    droplet growth.-   4. supersaturation is a function of flow rate, pressure and    temperature profile, which can be easily controlled and maintained.-   5. simple cylindrical geometry reduces size and minimizes buoyancy    (or other secondary flow) effects.    The principle of the CCN chambers has been validated by controlled    laboratory experiments and independent measurements.

In a more specific implementation, such a CCN chamber may have acylindrical column. The surfaces of the cylindrical column are wettedand exposed to an increasing and constant temperature gradient along thestream-wise axis constitutes the chamber volume. An air sample isintroduced at the center of the column and is surrounded by anaerosol-free humidified sheath flow. This configuration keeps the samplein a region of nearly uniform supersaturation and minimizes wall losses.The air then flows through the chamber, where CCN activate and grow intodroplets. An optical particle counter at the outlet detects and sizesall particles. Those particles larger than a threshold size (e.g., 1micron diameter) are considered CCN. This design maximizes the growthrate of activated droplets, thereby enhancing the performance of theinstrument. The temperature gradient and the flow through the columncontrol the supersaturation and may be modified to retrieve the CCNspectra.

The underlying mechanism of generating a supersaturation relies on thedifference in heat and mass diffusion as water vapor diffuses fasterthan heat (i.e., 0.25 m² s⁻¹ vs. 0.21 m² s⁻¹ at 294 K, 1 atm,respectively). The resulting supersaturation is largely dependent on thetemperature gradient and flow rate; while changes in pressure andtemperature affect rates of diffusion and exert a secondary effect.

The exemplary continuous-flow CCN chambers described here have acontinuous thermal gradient in the stream-wise direction of the air flowin the CCN chambers. For example, the thermal gradient may be amonotonic thermal gradient in the stream-wise direction of the air flowin the CCN chambers. Hence, the temperature of the CCN chamber changesmonotonically along the flow direction from the input end to the outputend. The temperature may monotonically increase along the flowdirection. In one implementation, the temperature of the cylindrical CCNchamber increases linearly along the flow direction from the input endto the output end. A linear temperature gradient yields a quasi-uniformsupersaturation along the centerline and maximizes the growth rate ofactivated droplets, thereby significantly enhancing the instrumentperformance. This monotonic streamwise temperature gradient not onlyimproves the performance of the CCN instrument but also makes the designsimpler.

FIG. 3 shows one exemplary implementation of a CCN instrument 300 havinga continuous-flow CCN chamber 310 with a monotonic thermal gradient inthe stream-wise direction of the air flow. The system 300 includes anaerosol flow line 320 to carry a desired sample flow to the chamber 310received from the Y-shaped inlet 321. The inlet 321 splits the input airflow into an aerosol flow in an aerosol flow pathway 322 and a sheathflow in a sheath flow pathway 324. A mass flow controller 325 and a pump326 are located in the sheath flow pathway 324 to produce the desiredvolumetric sheath flow to the chamber 310. A total particle filter 327is also in the sheath flow line 324 to remove all particles in thesheath flow. A volumetric flow element 323, e.g., a capillary flowelement coupled with a differential pressure transducer, may be placedin the aerosol flow pathway 322 to measure the volumetric flow rate ofthe aerosol flow which will be fed into the entrance section 311 toundergo the condensation in the chamber 310.

A water supply module 330 is also included in the system 300 tocirculate water through the chamber 310 to humidify the sheath flow andto wet the inner wall of the chamber. A water reservoir 331 is providedto supply water to the top of the chamber 310 and to uptake excess waterfrom the bottom of the chamber 310. A water pump 332 may be used tosupply the water from the reservoir 331 to a humidifier 328 in thesheath flow line 324 to humidify the sheath flow and supply the water tothe top of the chamber 310.

At the outlet of the chamber 310, a particle counter 340 collects outputair flow to measure the number of droplets (e.g., activated particles)in the aerosol flow. This particle counter 340 may be an opticalparticle counter (OPC) or other suitable counters to measure dropletsize and concentration. An air pump 350 may be coupled in the outputflow to induce the aerosol flow in the chamber 310 and to furthercontrol the flow rate.

The design in FIG. 3 has distinct features in comparison with other CCNchambers. For example, U.S. Pat. No. 6,330,060 to Flagan and Chuangdescribes a CCN chamber with alternating hot and coldtemperature-control sections to produce a spatially alternating thermalgradient along the flow direction. The supersaturation profile in thisalternating gradient CCN chamber is oscillating along the axis of thechamber in the direction of the flow. Hence, the residence time aparticle is exposed to a given supersaturation limits or even reversesgrowth of activated droplets.

FIG. 1 shows the alternating-gradient technique for repeating hot/coldsections in cylindrical symmetry. The upper graph illustrates thecontours of the supersaturation profile in the radial, r, andstreamwise, z, dimensions. The centerline is at r=0. The arrows show theparabolic velocity profile. The lower graph shows the alternatingtemperature gradient (bars) and the development of the supersaturationprofile near the centerline of the chamber (multiple oscillating lines).The minimum saturation ratio briefly drops below 1 (i.e., subsaturated)just after the cold sections. Droplets momentarily evaporate during thisperiod. Simulations of the alternating-gradient technique achieved onlymarginal performance using a fully-coupled model developed by Nenes,Chuang, Flagan, and Seinfeld in “A theoretical analysis of cloudcondensation nucleus (CCN) instruments,” J. Geophys. Res., 106,3449-3474 (2001).

In recognition of the above, the inventors conducted simulations on thealternating-gradient design and adjusted various instrument parametersto improve the performance. In a particular simulation, successivesections were heated slightly warmer than previous ones to prevent theairstream from equilibrating in a single heated section (i.e., thediffusion time of water vapor and heat to the center of the cylindricalchamber was longer than the residence time of the air parcel in a givenheated section). This pattern was extended along the column and produceda nearly constant centerline supersaturation with small oscillationsabout the mean value. Smaller simulated heated sections and a moreuniform increase in temperature resulted in a linear temperaturegradient and a smooth, nearly uniform centerline supersaturation.

FIG. 2 shows two plots of the simulation results based on the linearthermal-gradient technique for a cylindrical symmetry. The upper graphillustrates the contours of the supersaturation profile in the radialand streamwise dimensions. The centerline is at r=0. The lower graphshows the linear temperature gradient (lines with dot symbols) and thedevelopment of the uniform supersaturation profile near the centerlineof the chamber.

The design in FIG. 3 implements this linear thermal-gradient technique.Instead of controlling each of the multiple sections (i.e., 14 hot/coldsections) as in U.S. Pat. No. 6,330,060, a continuous temperaturegradient can be established by maintaining the temperature at a minimumof two locations 313A and 313B—at each end of the chamber 310. Otherfeatures and associated advantages of this design are described below.

In FIG. 3, the CCN chamber 310 is an elongated, cylindrical chamberhaving an entrance section 311 to receive an input air flow along thelongitudinal direction of the chamber and an output section 312 toexport the air flow for measurements of CCN. The chamber 310 may beoriented vertically with the entrance section 311 on the top and theoutput section 312 at the bottom. Thermal control units may be thermallycoupled to the ends of the chamber 310 to maintain a thermal gradient inthe chamber 310 along the flow direction. The thermal gradientestablishes a monotonic change in temperature along the wetted surfacein the flow direction, e.g., an increase in the wall temperature fromthe entrance section 311 to the output section 312. As illustrated, eachtemperature control unit 313A and 313B may include one or more thermalcontrol devices that are spatially separated along the chamber 310. Forexample, one or more thermal electric coolers (TECs) may be used toproduce the desired stream-wise monotonic thermal gradient in thechamber 310. Temperature sensors, such as thermocouples 314A and 314B,may be used to monitor the temperature at different locations within thechamber 310.

The system 300 in FIG. 3 provides a much needed improvement to themeasurements of CCN by providing a simple, yet, robust method ofprecisely generating a supersaturation in a cylinder. More detailedfeatures of the system 300 are now provided in the following sections.

Temperature Control

In the illustrated example, a vertical cylindrical column is used as theCCN growth chamber 310. The column's inner surfaces are wetted andexposed to an increasing temperature gradient along the streamwise,vertical axis. The dimensions of the column may be about 10.9 mm inradius. The length and wall thickness depend on the operating conditionsand may be about 360 mm in length with a wall thickness of about 8 mm.To generate a nearly-linear temperature gradient, the column's wallsshould be sufficiently thick such that heat transfer in the streamwiseaxis (within the wall) is much greater than the convective orevaporative heat losses to the sample aerosol flow and through theinsulation surrounding the column. Four thermal electric coolers (TECs)may surround the column on each end to maintain the prescribedtemperatures at their respective locations and maintain a desiredmonotonic temperature gradient along the axis of the chamber. The TECsmay be mounted on each side of a 34 mm×34 mm×38 mm block which securelyfits around the column. Heat-conductive silicon paste may be appliedbetween the TECs, block and the column to ensure proper heat transfer.Temperature measurements may be made at several locations shown in FIG.3. Thermocouples, resistance temperature detectors (RTDs), and othertemperature sensors may be used.

The top column temperature 314A operates near the temperaturesurrounding the instrument. The sheath flow that flows against the innerwall of the column 310 may be actively heated in the headblock 311 with,e.g., a resistance wire heater, to slightly above top column temperature(ca. 1 K) to prevent inadvertent activation of particles in the entranceportion of the column where the sample and sheath flows rejoin. Anotherresistance wire heater may be used to keep the optical particle counter340 (OPC) slightly warmer than the bottom-column temperature 314B toprevent condensation of water vapor on the optics and detector in theOPC.

Flow Control

An example of the CCN instrument 300 flow system is also shown in FIG.3. A Y-shaped inlet 321 splits the sample airstream into separateaerosol and sheath flows while minimizing impaction losses. The aerosolsample flows through a capillary and a differential pressure sensor(e.g., 5″ H₂O full-scale, temperature compensated Honeywell sensor) maymeasure the volumetric flow. Electrically conductive silicon tubing forthe aerosol flow line 324 is made as short as possible to minimizediffusion losses.

The sheath flow is directed through a total aerosol filter 327, flowmeter 325, pump 326, orifice, dead-volume, humidifier and heater beforebeing introduced into the headspace above the wetted column. A mesh 430,shown in FIG. 4, separates the headspace and the wetted column, and viaa slight pressure gradient uniformly distributes the aerosol-freehumidified sheath flow. Before rejoining with the aerosol flow, thesheath flow is accelerated into the growth column; once it has achievedfully developed, annular flow, the air sample is introduced at thecenter of the wetted column. This annular configuration keeps theaerosol flow in a region of nearly uniform supersaturation and confinesthe aerosol flow near the centerline to minimize wall losses. Themeasurements nominally used a 10-to-1 volumetric ratio for the sheathand aerosol flow rates which are also referred to as a sheath-to-aerosolratio (SAR) of 10. Configurations with SARs between 5 and 20 have beentested.

After the sheath and aerosol flows have been rejoined, the air thenflows vertically downward through the chamber (FIG. 4) and is exposed tothe increasing temperature gradient along the wetted surface within thechamber. Particles with a critical supersaturation less than thecenterline saturation ratio activate and grow into droplets. The lengthof the column and flow rate may be optimized to achieve sufficientlylarge particles to separate activated and unactivated droplets. Acollector cone 420 at the bottom of the chamber may focus the sample atthe bottom of the chamber and introduces the airstream into the OPC. Thecone 420 may have an included angle of 30° (i.e., 15° from the flowaxis), and its opening may be slightly smaller than the chamber diameterso that excess water drains along the walls without flooding the OPC340. A pump 350 downstream of the OPC 340 pulls the airstream throughthe instrument 300. Dead-volumes and orifices may be placed between eachof the pumps and the column to eliminate pressure oscillations in thegrowth chamber.

Optical Particle Counter

The optical particle counter (OPC) 340 in the system 300 in FIG. 3 mayemploy standard light scattering techniques to detect droplets at theoutlet of the growth column. The OPC is available commercially through,e.g., MetOne Intruments, Inc. (Grants Pass, Oreg., USA). The electronicsprocessor from MetOne counts and sizes the detector output into sixsize-selectable bins, which may be selected to be 0.5, 0.7, 1.0, 2.0,3.0, 6.0 μm diameter. The size cutoffs of the bins have been calibratedat MetOne and the smallest detectable particle size is 0.3 μm diameter.The number count of particles with diameters greater than the bin sizemay be exported via a RS-232 communication interface at 1 Hz.

A collection cone may be attached to the OPC to bring the sample intothe scattering volume with minimal bias to the droplet size spectra.Those droplets larger than a threshold (e.g., 1.0 μm diameter) may beconsidered CCN and comprise the CCN concentration.

Column Wetting

Various materials may be used on the inner surface of the chamber 310 inFIG. 3 to provide the desired wetting. For example, two layers of filterpaper 412 (Whatman 1) may be used to maintain a wetted inner surface ofthe chamber. A reservoir 331 below the column may supply the water to apump 332 (e.g. peristaltic pump), which pushes the water through ahumidifier 328 (e.g., from Perma-Pur Inc.) and into the top of thecolumn. Water is introduced through a radial band of small holes at topof the column for uniform distribution around the filter paper. Excesswater drains to the bottom of the column and back into the reservoir. Agun-barrel-type drill on the inner side of the conductive wall 410,under the wetted surface 412, may result in a more efficient wetting ofthe walls, as well as a more efficient way to drain excess liquid waterfrom the walls.

Electronics Interface

The electronic interface for the CCN instrument 300 in FIG. 3 may bedesigned for automatic acquisition and control of various operations.Its main parts may include a microprocessor or microcontroller such asan industry standard x86 compatible NEC V25 microcontroller, aninterface backplane and various extension boards, which interface theprocess controller to the various components. Because of its highmodularity, the system may be easily scaleable and, therefore, adaptableto the development of the CCN instrument. The interface is assembled ina standard 19″ rack-mounted container. Seven analog-to-digital (A/D)input channels (16 bit resolution, low noise, 50 Hz suppression) collecttemperature and differential pressure measurements. Eightdigital-to-analog (D/A) channels (12 bit, low noise, 50 Hz suppression)control the thermal electric coolers, pumps, and resistance heaters.Data storage capabilities on PCMCIA hard disks provide ample storagespace, and an RS-232 interface is also available to connect to secondaryhosts. An integrated menu-driven LCD and 6 keyboard user interfaceprovide an efficient, user-friendly interface for controlling variousparameters and assessing the performance of the instrument.

Design of Instrument

Flow and streamline constraints on the instrument in FIG. 3 are nowdescribed. Before constructing the instrument, extensive simulationsusing the fully coupled model of various parameters (i.e., columndimensions, heating rates, and flow rates) placed operational anddimensional constraints on the instrument design. In particular, specialattention was devoted to buoyancy-related issues that affect theinstrument's performance. Earlier attempts to produce a similarinstrument failed because of degraded performance resulting, in part,from secondary (buoyancy) flows that developed using large temperaturegradients and low flow rates. Parameters such as column radius,temperature, temperature gradient and flow rate must be carefullyselected to ensure proper performance.

One important dimension of the instrument is the column radius, whichdictates the droplet's residence time in the column (hence, allowablegrowth). Instrument performance was measured by determining the ratio ofthe maximum radial change in the streamlines (maximum amount of changein streamlines perpendicular to the flow) to the radius of the column.Simulations were performed by using the fully coupled model that coverthe operating range of the CCN instrument as shown in FIG. 6. For allsimulated conditions, even at low flows and large temperature gradients,there is a notable increase in the deviation of streamlines as theradius increases above 12 mm.

In addition, buoyancy effects may also set limits on the maximumtemperature gradients. As one might expect, large temperature gradientsalso drive unwanted convective motion as shown by the onset ofconvective activity at smaller radii in FIG. 6; and ultimately, limitingthe maximum supersaturation at a given flow rate. Simulations indicatethat the maximum supersaturation for the current configuration is ca. 3%at 1 liter per minute, which is sufficiently high for ambientmeasurements.

At small supersaturations, low flow rates may be needed to provide theresidence time required for adequate growth for detection ordistinguishing the activated droplets from deliquesced particles.However, low flow rates are more susceptible to buoyancy effects and areultimately limited by the terminal settling velocity of growingdroplets. We operate the CCN instrument with flow in the downwardvertical direction to eliminate problems associated with the suspensionof larger droplets in the sample flow. The above-described instrumentbased on FIG. 3 was tested to be operable for flow rates between 0.3 and1 lpm.

The instrument in FIG. 3 may be capable of producing supersaturationsless than 0.1%. In this operating range, it is important to distinguishactivated droplets from deliquesced particles for identifying CCNaccording to the Köhler curve definition. As discussion earlier, aparticle is consider activated when the droplet grows beyond itscritical diameter, D_(pc); where it will continue to grow as long asthere is sufficient water vapor. This growth process, however, isdiffusion limited and a finite time is required to grow the droplets.Distinguishing activated from unactivated droplets is relativelystraightforward for small aerosol particles whose critical diameters aresignificantly less than the 1 μm threshold. However, as particles becomelarger, the critical size approaches the detection size and activatedand unactivated droplets are not readily identified. To obtainmeasurements of CCN at lower supersaturations, we need to increase thethreshold to larger sizes; however, there is a trade-off between thisthreshold and the residence time needed for adequate growth.

An analytical solution of maintaining a constant supersaturation in acylindrical column is described below for an ideal fully developed,laminar flow. This solution determines the supersaturation at any pointwithin the chamber. The generalized conservation principle forsteady-state laminar flow in axisymmetric coordinates can be expressedas $\begin{matrix}{{{u\frac{\partial X}{\partial z}} + {v\frac{\partial X}{\partial r}}} = {\frac{\alpha}{r}\frac{\partial}{\partial r}\left( {r\frac{\partial X}{\partial r}} \right)}} & (1)\end{matrix}$where X is either temperature or water vapor; u and v are streamvelocities in the streamwise, z, and radial, r, directions, and α is thethermal or water vapor diffusivity in air. The terms on the left-handside account for fluid motion; and those on the right-hand side accountfor diffusion. The solution to the above equation is readily obtainedfor fully developed flow at steady state with no convection, no slipboundary, and constant surface heat flux. The equation reduces to$\begin{matrix}{{X\quad(r)} = {X_{s} - {\frac{2u_{m}r_{o}^{2}}{\alpha}{\left( \frac{\mathbb{d}X}{\mathbb{d}z} \right)\left\lbrack {\frac{3}{16} + {\frac{1}{16}\left( \frac{r}{r_{o}} \right)} - {\frac{1}{4}\left( \frac{r}{r_{o}} \right)^{2}}} \right\rbrack}}}} & (2)\end{matrix}$

where X_(s) is the surface temperature or water vapor pressure, u_(m) isthe mean flow velocity through the tube, and r_(o) is the radius of thecolumn. Obtaining the value of the temperature profile, dT/dz, istrivial in the implementation of a linear thermal gradient along the zaxis. However, the equilibrium vapor pressure, C, increases withtemperature and the expression for dC/dz is shown to be $\begin{matrix}{\frac{\mathbb{d}C}{\mathbb{d}z} = {\gamma_{1} \cdot {\exp\quad\left\lbrack \frac{\gamma_{1} \cdot \left( {T - T_{o}} \right)}{\left( {T - \gamma_{2}} \right)} \right\rbrack} \cdot \frac{\mathbb{d}T}{\mathbb{d}z} \cdot \left( \frac{\gamma_{1} \cdot \left( {T - T_{o}} \right)}{\left( {T - \gamma_{2}} \right)} \right)^{2}}} & (3)\end{matrix}$where T_(o) is 273.15 K, and γ₁, γ₂ and γ₃ are constants (6.113×10⁻³,17.67 and 26.95, respectively). The saturation (%), which decreasesslightly along the z axis, is defined as $\begin{matrix}{S = {{\left( {\frac{C}{C_{eq}} - 1} \right) \cdot 100}\quad\%}} & (4)\end{matrix}$where C_(eq) is the equilibrium water vapor pressure at a giventemperature within the column.

The validity of this simplified analytical approximation has beenverified through model comparisons and laboratory experiments. Acomparison of the simplified model and the fully coupled model suggeststhat the main features of the dual diffusion in laminar flow in acylindrical tube have been captured. The main differences between thetwo models arise from transient non-laminar flows and convectivemotions. The longer hydrodynamic entry length at larger flows impedesthe development of a parabolic laminar profile causing the deviations insuperaturation predictions with respect to the flow rate. Differences inthe diffusivities and density from changes in temperature are also notconsidered in the simplified analysis.

The analytical steady-state and fully-coupled transient modelsimulations present the characteristics of the CCN instrument andillustrate the dependence of the supersaturation at various operatingconditions. FIG. 5 illustrates this dependence on several importantvariables, including flow, temperature gradient, pressure andtemperature. The flow rate and temperature gradient exert the largestinfluence on determining the supersaturation; where the rate of changein supersaturation is ca. 0.06% per 100 cm³ min⁻¹ change in flow rateand 0.10% per K m⁻¹ change in temperature gradient. The absolutepressure and entrance temperature of the column also influence thesupersaturation by changing the rate of diffusion; however, the effectsof absolute pressure and temperature are modest at ca. 0.03% per 100mbar and 0.034% per 10 K, respectively. Nonetheless, slight changes inS_(v) are expected during vertical profiles in airborne measurements. Atypical profile may cover a range between 600 and 700 mbar, whichcorresponds to a ca. 0.2% change in S_(v). To maintain a constant S_(v),one may vary other parameters such as the temperature gradient or flowrate to compensate the pressure dependence in S_(v).

Additional modifications to the current design include but are notlimited to the following:

-   1. Capillary flow elements simplify measurements of volumetric flow    rates compared to mass flow meters, because they eliminate the need    for pressure and temperature compensation during flight missions.    The differential pressure sensors also require less power than mass    flow controllers.-   2. The wall thickness of the chamber needs to be minimized, yet    sufficiently thick such that heat conduction in the longitudinal    direction is greater than convective and evaporative losses from the    sides.-   3. Maintaining a wet column has been a challenge for CCN    instruments. Currently, the Whatman filter paper is used to line the    inside of the chamber. Other materials may also be used for    maintaining desired wet surfaces in the column. For example, porous    ceramics, such as alumina bisque, may also serve as the wetting    medium. The thermal conductivity of alumina bisque ceramics (4.3 W    K⁻¹ m⁻¹) is much higher than that of paper (0.18 W K⁻¹ m⁻¹), leading    to smaller temperature differences between measured and    wetted-surface temperatures. The use of ceramics also reduces the    need of accessing the internal parts of the instruments making it    more robust and “field-friendly.”-   4. Since the supersaturation exhibits a flow-rate and pressure    dependence, feedback on the temperature and/or flow control must    compensate for changes in pressure during vertical profiles to    maintain a fixed supersaturation.

In addition, other implementations may be possible. For example, thetemperature profile may increase or decrease along the streamwise axisand need not be linear. A non-linear temperature profile may apply tocertain applications.

Also, the temperature profile may not necessarily be monotonic along theentire length of the column. For example, additional temperature controlmay be added such that the first part of the column may experience anincreasing temperature gradient, whereas the latter part of the columnmay experience a decreasing or no temperature gradient.

The instrument may comprise at least two or more CCN columns withdifferent particle counters to perform simultaneous measurements inparallel on air samples taken at the same locale. These columns may beat different lengths, or may be exposed to different operatingconditions such as temperature gradients, internal pressures, differentflow rates, and/or different media.

The CCN chamber designs here may apply to media other than the typicalthe medium of air and water and in general may apply to measurements inany media that exhibit different rates of mass and thermal diffusivity.The construction of the instrument and column may include materialsother than aluminum and the walls of the chamber may consist ofmaterials other than filter paper. Detectors or other devices other thanan optical particle counter may also be used depending on theapplication. Air flow pattern in the CCN chamber other than annular flowthrough the column may also be acceptable. Furthermore, the instrumentneed not necessarily be positioned in the vertical direction.

1. A device, comprising: a cloud condensation nuclei chamber having aninput to receive an aerosol flow, a region of supersaturation to growcloud condensation nuclei, and an output to export the aerosol flow; anda thermal control engaged to said chamber to produce a monotonic thermalprofile in a stream-wise direction of the aerosol flow from said inputto said output in said chamber.
 2. The device as in claim 1, wherein atemperature in said chamber monotonically increases along the aerosolflow.
 3. The device as in claim 2, wherein the temperature of saidchamber linearly increases along the aerosol flow.
 4. The device as inclaim 2, wherein the temperature of said chamber nonlinearly increasesalong the aerosol flow.
 5. The device as in claim 1, further comprisinga flow control mechanism to split an air sample flow into the aerosolflow and a sheath flow, wherein the sheath flow is directed to flowalong inner surfaces of said chamber to keep the aerosol flow away fromthe inner surfaces.
 6. The device as in claim 5, wherein the sheath flowhas a sheath flow rate higher than a flow rate of the aerosol flow. 7.The device as in claim 1, wherein said chamber has a cylindrical shapeto direct the aerosol flow along an axis of the cylinder.
 8. The deviceas in claim 1, wherein said chamber is oriented vertically to receivethe aerosol flow from the top and the expert the aerosol flow from thebottom.
 9. A cloud condensation nuclei measuring apparatus, comprising:a chamber to receive an air sample and to keep said air sample in aregion of supersaturation within a specified range; a heating systemproviding an increasing temperature gradient along the axis of saidchamber in the direction of flow; and a particle counter coupled to saidchamber to measure particles in said air sample output by said chamberand to provide a count indicative of particles within a selected sizerange.
 10. The apparatus as in claim 9, further comprising a flowcontrol mechanism to provide a sheath flow around the air sample in saidchamber and to keep the air sample away from side walls of said chamber.11. The apparatus as in claim 10, wherein a ratio of a flow rate of thesheath flow over a flow rate of the air sample is controlled betweenabout 5 and
 20. 12. The apparatus as in claim 10, further comprising aheating element to heat the sheath flow at a temperature above atemperature of an end of the said chamber that receives the air sample.13. The apparatus as in claim 9, wherein said chamber has a wetted innersurface.
 14. The apparatus as in claim 13, wherein said chamber has alayer of a filter paper on the wetted inner surface.
 15. The apparatusas in claim 13, wherein said chamber has a layer of a porous ceramicmaterial on the wetted inner surface.
 16. The apparatus as in claim 9,wherein said particle counter includes an optical particle counter. 17.The apparatus as in claim 9, wherein temperatures along the axis of saidchamber linearly increase.
 18. The apparatus as in claim 9, furthercomprising a second chamber to receive a second air sample and to keepsaid second air sample in a region of supersaturation within a specifiedrange; a second heating system providing an increasing temperaturegradient along the axis of said second chamber in the direction of flow;and a second particle counter to measure particles in said second airsample output from said second chamber and to provide a count indicativeof particles within a selected size range.
 19. A thermal gradientdiffusion chamber for inclusion in a cloud condensation nucleimeasurement apparatus comprising a heat source to create an increasingtemperature gradient in the direction of flow of an air sample in saidchamber.
 20. The chamber in claim 19, wherein said chamber has a wettedinner surface.
 21. The apparatus as in claim 19, wherein temperaturesalong the axis of said chamber linearly increase.
 22. A method forconditioning a sample in a cloud condensation nuclei measurementapparatus, comprising: subjecting a sample passing through a column; andsubjecting said sample to an increasing temperature gradient in thedirection of sample flow.
 23. The method as in claim 22, furthercomprising using a sheath flow around the sample flow to keep the sampleflow away from inner surfaces of the column.
 24. The method as in claim22, further comprising maintaining inner surfaces of the column wet withwater.
 25. A method, comprising: directing an aerosol flow through acloud condensation nuclei chamber to grow particles due to condensationfrom supersatuation; and controlling a temperature profile of thechamber along the aerosol flow to produce a nearly constantsupersaturation along the chamber.
 26. The method as in claim 25,further comprising providing a sheath flow around the aerosol flow toreduce particle loss caused by contact of particles in the aerosol flowand inner surfaces of the chamber.
 27. The method as in claim 25,wherein a temperature of the chamber increases monotonically along thedirection of the aerosol flow.